Triptolide, a compound derived from the Chinese medicinal plant Tripterygium wilfordii (TW), has been shown to have significant immunosuppressive activity, e.g. in the treatment of autoimmune disease, and in treating or preventing transplantation rejection, including the treatment of graft-versus-host disease (GVHD), a condition in which transplanted marrow cells attack the recipient's cells. Various prodrugs and other analogs of triptolide have also shown such activity. See, for example, U.S. Pat. Nos. 4,005,108, 5,294,443, 5,843,452, 5,648,376, and 5,962,516, which are incorporated herein by reference. Triptolide and its prodrugs and other analogs have also shown significant anticancer activity, including reduction of solid tumors in vivo. See, for example, co-owned U.S. Pat. No. 6,620,843, which is incorporated herein by reference, and Fidler et al., 2003.